2.1 Autoimmune Disorders
Host immune responses are commonly classified into two distinguishable groups, cellular and humoral. Cellular immunity is mediated by T lymphocytes or T cells and protects against virally infected cells, fungi, parasites, and foreign tissue. Humoral immunity, which is mediated by B lymphocytes or B cells through the production of antibodies, is most effective against bacterial infections and the extracellular phases of viral infections. D. Voet & J. Voet, Biochemistry 1208 (2d ed., Wiley 1995).
Cellular immune response cascades lead to the destruction of antigens through: 1) the uptake of antigens by cellular macrophages or antigen presenting cells, 2) the processing or fragmentation of the antigen within the macrophage, 3) the association of the fragmented antigen with cell-surface proteins known as the major histocompatibility complex (“MHC”) proteins and 4) binding of the MHC protein/antigen complex by T cells that are induced to propagate, thereby illiciting an effective immune response against the specific antigen. In autoimmune disorders, this cellular immune response cascade recognizes self-derived species as antigens, effectively leading to the destruction of host peptides, cells, and tissues. The process by which the cellular immune system recognizes and initiates a response to antigens, both foreign and self, has been a focus of much research in recent years.
MHC proteins have been classified into two groups, referred to as Class I and Class II MHC proteins, that are structurally and functionally similar. D. Voet & J. Voet, Id. Macrophages exhibiting Class I MHC proteins or Class II MHC proteins that are complexed with an antigen on their surface are bound by cytotoxic T cells or helper T cells, respectively. As a result of this binding event, T cells are induced to proliferate and trigger an immune response against the antigen. The role of MHC proteins is to present the antigen on the surface of the cell so that they can be recognized by T cells. In humans, the Class I MHC proteins are encoded by three separate genetic loci, HLA-A, HLA-B, and HLA-C. There are also three heterodimeric human Class II MHC proteins whose alpha and beta chains are encoded by genes designated HLA-DP, HLA-DQ, and HLA-DR. Both Class I and Class II MHC genes are highly polymorphic, giving rise to the variance between individuals in the population. Id. Because of the key role of the antigen/MHC complex in the activation of T cells, inhibition of antigen binding to MHC molecules has been a goal of research in autoimmune diseases. Id.
In autoimmune diseases, inappropriate triggering of T cell responses by MHC molecule—“self antigen” complexes leads to destruction of normal tissues. Individuals inherit MHC genes of the HLA-DR, -DP, and -DQ haplotypes, and these have been linked to specific autoimmune diseases and autoantigens such as multiple sclerosis and rheumatoid arthritis. In each of these cases, patients diagnosed with the autoimmune disease carry an associated MHC gene. In rheumatoid arthritis, over 80% of patients have either HLA-DR1 or DR4 genes; in multiple sclerosis, ca. 70% have HLA-DR2.
Design of inhibitors of the cellular immune response has been a fundamental goal of research that aims to prevent T cell proliferation in autoimmune disease. Yusuf-Makagiansar et. al. (2002) Med Res. Rev. 22(2):146-167, Adorini, et al., (1988) Nature, 334, 623-625. For example, Astra-Zeneca has a partially stabilized, large peptide analog (ZD 2315) in phase II clinical trials that binds to DR1/4 based on these principles. The compound was shown to be active in vivo in mouse models. Cytel, Inc. developed a partially stabilized peptide (a(Cha)AAAKTAAAAa-NH2) (SEQ ID NO.: 106) that binds DR molecules, but did not supress T cell proliferation in response to protein antigens, and also did not show activity in animal models of autoimmune disease (Lamont, et at. (1990), J. Immnol. 144, 2493-2498; Ishioka, et al., (1994) J. Immunol. 152, 4310-4319. The lack of cellular activity was thought to be inherent to peptides, because they are (a) susceptible to rapid exchange within the endosomal loading compartment in the presence of HLA-DM, and (b) are prone to cleavage by proteolyic enzymes of the cathepsin class that reside in the endosome to process protein antigens.
The interactions between peptides and MHC molecules have been defined in a series of high resolution crystal structures. Stem et al. (1994) Nature 368:215-221; Smith et al. (1998) J. Exp. Med. 188:1511-1520. The general observations include the extended, poly(proline) II helical conformation of the peptide backbone, the presence of pockets (that bind so-called anchor residues) along the chain, and networks of hydrogen bonds between the peptide backbone and side chains of the MHC molecules that line the binding site.
In other work, requirements for peptide binding have been assessed using structure-activity studies (Hammer, J. et al. (1993) Cell 74:197-203) withpeptide phage display libraries.
2.2 Multiple Sclerosis
Multiple sclerosis is a chronic demyelinating autoimmune disease of the central nervous system that afflicts over 2 million patients worldwide, with approximately 350,000 patients in the U.S., with an average of 200 new patients diagnosed weekly. With the exception of trauma, multiple sclerosis is the leading cause of neurologic disability in early to middle adulthood. The disease is typically progressive, incapacitating, and affects multiple body systems. Accordingly, there remains a need for an effective treatment for multiple sclerosis in addition to other autoimmune diseases.
In multiple sclerosis, antigenic peptides derived from the myelin nerve sheath bind to the MHC class II molecule HLA-DR2. The specific allele, MHC class II HLA-DR2 (DRA*0101/DRB1*1501), has been closely associated with the multiple sclerosis. In people of northern European descent, approximately 70% of multiple sclerosis patients carry the HLA-DR2 gene. Giordano, M. et al. (2002) Am. J. Pharmacogenomics 2:37-58. Accordingly, compounds with the ability to inhibit antigen binding by the MHC class II HLA-DR2 are being sought as effective therapeutics for treating or preventing multiple sclerosis.
An additional study focused on the structural requirements for binding of a compound to DR2 molecules by probing peptides that comprise myelin basic protein (“MBP”). Wucherpfennig et al. (1994) J. Exp. Med. 179:279-290. The immunodominant MBP (84-102) peptide was found to bind with high affinity to DRB1*1501 and DRB5*0101 molecules of the disease-associated DR2 haplotype. Other peptide segments that overlapped with this peptide were also critical for binding to these molecules. It was demonstrated that hydrophobic residues (Val189 and Phe92) in the MBP (88-95) segment were critical for peptide binding to DRB1*1501 molecules and that hydrophobic and charged residues (Phe92, Lys93) in the MBP(89-101/102) sequence contributed to DRB5*0101 binding.
There remains a need for prophylactic or therapeutic drugs that can be used to treat or prevent autoimmune diseases, in particular multiple sclerosis. This invention addresses these and other needs in the pharmaceutical area.